High Performance n-type PbSe-Cu2Se Thermoelectrics through

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High Performance n-type PbSe-CuSe Thermoelectrics through Conduction Band Engineering and Phonon Softening Chongjian Zhou, Yuan Yu, Yong Kyu Lee, Oana Cojocaru-Miredin, Byeongjun Yoo, Sung-Pyo Cho, Jino Im, Matthias Wuttig, Taeghwan Hyeon, and In Chung J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b10448 • Publication Date (Web): 22 Oct 2018 Downloaded from http://pubs.acs.org on October 22, 2018

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Journal of the American Chemical Society

High

Performance

n-type

PbSe-Cu2Se

Thermoelectrics

through Conduction Band Engineering and Phonon Softening Chongjian Zhou,†,‡ Yuan Yu,ᴨ Yong Kyu Lee,†,‡ Oana Cojocaru-Mirédin,ᴨ Byeongjun Yoo,†,‡ Sung-Pyo Cho,§ Jino Im, ∥ Matthias Wuttig,ᴨ,∆ Taeghwan Hyeon,†,‡ and In Chung†,‡,,*

†Center

for Nanoparticle Research, Institute for Basic Science (IBS), Seoul 08826,

Republic of Korea ‡School

of Chemical and Biological Engineering, Institute of Engineering Research, and

§National

Center for Inter-University Research Facilities, Seoul National University,

Seoul 08826, Republic of Korea ∥Chemical

Data-Driven Research Center, Korea Research Institute of Chemical

Technology, Daejeon 34114, Republic of Korea ᴨInstitute

of Physics (IA), RWTH Aachen University, 52056 Aachen, Germany

∆JARA-FIT

Institute Green-IT, RWTH Aachen University and Forschungszentrum Jülich,

52056 Aachen, Germany * To whom correspondence should be addressed: [email protected]

ABSTRACT From a structural and economic perspective, tellurium-free PbSe can be an attractive alternative to its much expensive isostructural analogue of PbTe for intermediate 1 ACS Paragon Plus Environment

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temperature power generation. Here we report that PbSe0.998Br0.002-2%Cu2Se exhibits record high peak ZT ~1.8 at 723 K and average ZT ~1.1 between 300 K and 823 K to date for all previously reported n- and p-type PbSe-based materials as well as telluriumfree n-type polycrystalline materials. These even rival the highest reported values for ntype PbTe-based materials. Cu atoms not only enhance charge transport properties but also depress thermal conductivity of n-type PbSe. Cu2Se doping flattens the edge of conduction band of PbSe, increases the effective mass of charge carriers, and enlarges energy band gap, which collectively improve Seebeck coefficient markedly. This is the first example of manipulating electronic conduction band to enhance thermoelectric properties of n-type PbSe. Concurrently, it increases carrier concentration with nearly no loss in carrier mobility, even increasing electrical conductivity above ~423 K. Resulting power factor is ultrahigh, reaching ~21 − 26 μW cm−1 K−2 over a wide range of temperature from ~423 − 723 K. Cu2Se doping substantially reduces lattice thermal conductivity to ~0.4 W m−1 K−1 at 773 K, approaching its theoretical amorphous limit. According to first principles calculations, the achieved ultralow value can be attributed to remarkable acoustic phonon softening at low frequency region.

■ Introduction A thermoelectric (TE) module is a solid-state electronic device comprising numerous junctions of n- and p-type conducting solids.1–4 A heat source near such junctions renders charge carriers to move, thereby generating electricity spontaneously.5– 11

Accordingly, TE technology can be an efficient and environmentally friendly means of

recovering a huge amount of ubiquitous waste heat,12–15 which is approximately 66% of total energy input.16 This heat-recovery electronic system operates with no noise and 2 ACS Paragon Plus Environment

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vibration, giving high mechanical and environmental stability without releasing undesirable chemical residues like greenhouse gases. Because there is no restriction on the size and shape of device structures, TE modules are highly promising power generators for micro- and flexible electronics1,2 as well as conventional chemical plants and automobiles.17 The performance of a TE material is commonly evaluated by the dimensionless figure of merit, ZT = S2σT/tot, where S is the Seebeck coefficient, σ is the electrical conductivity, their product S2σ is the power factor (PF), T is the absolute temperature, and tot is the total thermal conductivity contributed from both charge carriers (ele) and lattice vibration (lat).12–15,18–24 PbTe-based compounds are the most efficient TE materials in the intermediate temperature range (600 − 900 K),25–33 in which approximately 90% of all waste heat occurs in the United States.34 However, only 0.001 ppm abundance of Te in the Earth’s crust hinders their broad applications.35 PbSe can be an attractive Te-free TE system, given its same crystal structure as PbTe and the fifty-fold larger natural abundance of Se than Te.35 Its higher melting point can give improved thermal stability, enabling higher TE operation temperatures. Indeed, p-type PbSe systems have been greatly improved by the multiple performance-enhancing strategies recently.36–38 For example, PbSe alloyed with CdS exhibits ZT ~1.6 at 923 K,37 the highest reported value so far both for p- and ntype PbSe.36–41 Its performance is mainly attributed to energetic convergence of closely lying light hole L and heavy hole  bands, leading to the enhancement of Seebeck coefficient and power factor, as well as hierarchical nanostructuring that results in the reduction of lattice thermal conductivity. In contrast, such band engineering has not been reported for n-type PbSe because of its absence of closely lying second conduction band. 3 ACS Paragon Plus Environment


As a result, strategies for enhancing Seebeck coefficient and power factor have long been elusive for n-type PbSe, and instead its TE performance has been almost exclusively improved by decreasing lat through enhancing phonon scattering. Examples are Pb0.9975Sb0.0025Se with extensive nanoscale precipitates (ZT ~1.5 at 830 K),39 Pb0.95Sb0.033Se with vacancy-induced dense dislocations (ZT ~1.6 at 900 K),42 and PbCu0.00375Se with hierarchical phonon scattering (ZT ~1.45 at 813 K).43 Recently, Pb0.95Sb0.033Se0.6Te0.4 (ZT ~1.5 at 823 K) shows not only enhanced power factor but also suppressed thermal conductivity by decoupling carrier mobility and lat, but its Seebeck coefficient was not enhanced.44 Here we report that the high extent of Cu2Se can be incorporated into the PbSe lattices, which can simultaneously enhance electrical transport properties and suppress thermal conductivity, achieving highly efficient n-type PbSe thermoelectrics. First, Cu2Se doping modulates conduction band minimum to be flat, makes charge carriers heavier, and broadens the energy band gap. These induced factors synergistically improve Seebeck coefficient significantly. This is the first example for modulating electronic conduction band to improve Seebeck coefficient for n-type PbSe materials, which has been failed so far due to the single conduction band nature of PbSe. Despite the enhanced Seebeck coefficient, the 2% Cu2Se doping even slightly increases electrical conductivity in the mid to high temperature range due to significantly increased carrier concentration and no damage in carrier mobility. Accordingly, average power factor from 573 to 773 K is remarkably high at ~22 μW cm−1 K−2. Second, Cu2Se doping markedly depresses low frequency phonon transport to give ultralow lattice thermal conductivity of 0.40 W m−1 K−1 at 723 K. As a consequence, a peak ZT (ZTmax) of ~1.8 at 723 K and average ZT

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(ZTave) of ~1.1 from 300 to 823 K can be achieved for n-type PbSe, outperforming any class of Te-free polycrystalline n-type thermoelectrics. The achieved ZTmax and ZTave reach the highest values ever reported for n-type PbTe.30,32,45,46 These results open a new generation of tellurium-free thermoelectrics for the intermediate temperature range power generation.

■ Results and Discussion 1. Design Principle and Crystal Structure PbSe crystallizes in the cubic rock-salt structure with an Fm-3m space group. Its unit cell contains eight symmetrically identical interstitial voids with the radius of ~1.07 Å, surrounded by four respective Pb and Se atoms (Figure 1a). Cu atom can be readily introduced into such voids considering the ionic radius of ~0.71 Å in a tetrahedron of Se ligands.47 This can induce favorable mechanisms for thermal and charge carrier transport properties of PbSe. First, introduction of smaller and weakly bound guest species into cages frequently induces a reduction in the lattice thermal conductivity (lat) by phonon scattering due to their rattling vibration with strong lattice anharmonicity as well as atomic scale point defects.48–54 Second, electrostatic repulsion between Pb and Cu cations is expected to disrupt the ordered PbSe lattices locally, thereby further suppressing lat.55 Third, highly mobile characteristics of Cu cations may also contribute to simultaneously improving electrical transport properties and reducing lattice thermal conductivity. All samples with the compositions of PbSe0.998Br0.002-x%Cu2Se (x = 0 – 7) form the rock-salt structure without impurities within the detection limit of laboratory X-ray diffraction despite the high doping level of Cu2Se up to 7% (Figure 1b). The unit cell 5 ACS Paragon Plus Environment


dimension increases nonlinearly with the increased mole fraction of Cu2Se doping up to x = 7, indicating that a majority of Cu occupies the interstitial holes in the PbSe lattices rather than replaces Pb (Figure 1c). The increased cell volume can be attributed to the electrostatic repulsion between Cu and adjacent Pb. The observed solubility of Cu2Se in PbSe0.998Br0.002 is remarkably high in comparison with the previous report that a limited amount of up to 0.35% Cu could be inserted into interstices in the PbSe lattices to form PbCu0.0035Se and a majority of Cu is isolated as nanoscale precipitates of Cu-rich selenides.43 Electronic absorption spectra were obtained for Br-free PbSe-x%Cu2Se samples because of severe spectroscopic interference by free carriers for PbSe0.998Br0.002x%Cu2Se samples. The value of the energy band gap (Eg) may not be affected because Br does not change electronic band structure. The results reveal that Cu2Se doping enlarges the Eg from 0.19 eV for pristine PbSe to 0.22 and 0.24 eV for the x = 2 and 3 samples, respectively (Figure 1d).

Figure 1. (a) Illustration of the PbSe structure with Cu atoms residing at interstitial voids. It is at the center of the cube comprising each four alternating Pb and Se atoms. From another view, it is shared by Pb tetrahedron (blue) and Se tetrahedron (orange), which interpenetrate with each other. (b) Powder XRD patterns and (c) lattice parameter with respect to the Cu2Se content for the PbSe0.998Br0.002-x%Cu2Se (x = 0 – 7) samples. (d) Electronic absorption spectra of the PbSe-x%Cu2Se samples (x = 0, 1, 2 and 3). 6 ACS Paragon Plus Environment

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2. Atomic and Nanoscale Structures After observing the unusually high solubility of Cu2Se in PbSe0.998Br0.002, we investigated atomic resolution structure of the PbSe0.998Br0.002-2%Cu2Se sample using spherical aberration-corrected scanning transmission electron microscope (Cs-corrected STEM) equipped with an energy dispersive spectroscopy (EDS) detector. All STEM images were taken parallel to the zone axis because respective Pb and Se atoms are linearly aligned along this direction so that each atomic species is clearly distinguishable by its signal intensity in high-angle annular dark field (HAADF) STEM images. Note that the intensity of HAADF-STEM image is roughly proportional to the square of the atomic number (Z). Representative medium-magnification HAADF-STEM image shows that a high degree of elastically strained stripes, enclosed by dotted lines, with a length of ~ 10 nm and a width less than 2 nm is embedded in the surrounding PbSe matrix (Figure 2a). Typical high-magnification atomic resolution HAADF-STEM image focusing on the stripe (marked by the yellow ellipsoid) is shown in Figure 2b. The bigger and brighter spheres can be assigned to Pb atoms and the smaller and faint spheres between two neighboring Pb atoms are Se atoms. The most striking feature is that weak satellite signals, indicated by the magenta arrows, are observed near Pb atoms inside the stripe. This reveals that Pb atoms in the stripe are slightly off-centered from the regular octahedral site in the rock-salt structure along the zone axis possibly due to their Coulomb repulsion with interstitial Cu atoms. Resultant modulated structure can increase bonding anharmonicity, which is highly effective phonon scattering mechanism.56 We will discuss its influence on lattice thermal conductivity later.



Despite the off-centered Pb and inserted Cu atoms, no dislocations occur inside and around the stripes, confirming elastic strains around them.26 The shear strain map profile (εxy) for Figure 2b is derived by geometric phase analysis (GPA), which is a semiquantitative process for high quality TEM images to give spatially distributed strain fields (inset). This result verifies that elastic strains are accumulated inside the stripe and are reduced significantly in the surrounding matrix. Such strains are probably caused by the presence of displaced Pb atoms and interstitial Cu atoms and resulting elastic distortion of the lattices. The observed highly coherent nanoscale structures in the matrix can be efficient for phonon scattering with a minimal deterioration in charge carrier transport.26 Much lighter Cu atoms (Z = 29) could not be clearly observed in Figure 2b because HAADF-STEM is highly sensitive to the atomic number. Accordingly, the corresponding annular bright field (ABF)-STEM image was taken (Figure 2c). It clearly shows that bright and small atoms occupy the interstices in the PbSe lattices, which are presumably Cu atoms. The corresponding fast Fourier transform image (FFT, inset of Figure 2c), which includes both the stripes and matrix, shows a single set of the diffraction pattern that corresponds to the rock-salt structure along the zone axis. This indicates that Cu atoms readily dissolve into the interstices of the PbSe lattices, avoiding segregation of second phase. It also follows that the stripes and the matrix have similar structure and lattice dimension, forming a coherent interface with the crystallographic alignment. The number density of Cu atom is much higher at and around the strained stripes than in the surrounding matrix. This observation indicates that introduction of Cu and resulting displacement of Pb induce the generation of strained and


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coherent nanoscale stripes. On the contrary, the previous reports on PbCu0.0075Se43 and PbTe-5.5%Cu2Te32 revealed the nanoscale precipitation of Cu-rich selenide43 and Cu2Te,32 respectively. To directly observe elemental arrangements in the nanoscale stripe, we conducted atomic resolution elemental mapping by STEM-EDS. The location of Pb and Se atoms is directly verified by the enlarged HAADF-STEM image and the corresponding elemental mapping down the zone axis taken on the seemingly defect-free (Cu-free) region (Figures 2d, e and S1), which matches well with the known PbSe structure (Figure 2f). Note that pristine PbQ (Q = Se, Te) is intrinsic p-type semiconductor due to Pb vacancies.28,37 Indeed, the recent paper on PbTe-5.5%Cu2Te showed the presence of Pb vacancies and Cu atoms occupying such spaces.32 However, similar results are not observed in the PbSe0.998Br0.002-2%Cu2Se sample. Given that pristine PbSe single crystals exhibit carrier concentration of ~ 4.28 × 1018 cm−3,57 the concentration for intrinsic Pb vacancies is only ~0.075 at%, which would be too low to be detected by our STEM. However, the possibility of Cu atoms at the Pb sites cannot be totally excluded given our temperature-variant Hall carrier concentration data for the sample with low 1% Cu2Se doping, as will be discussed later. Elemental mapping scanned on the stripe region clearly confirms the presence of slightly displaced Pb atoms and interstitial Cu atoms in the PbSe lattices by recording direct EDS signals from each atom (Figures 2g and S2), consistent with the results of our HAADF- and ABF-STEM. Figure 2h represents the schematic local structure based on the HAADF-STEM and STEM-EDS results in Figure 2g. We further analyzed nanostructures embedded in the PbSe0.998Br0.002-2%Cu2Se sample by atom-probe tomography (APT) measurement, which offers the three-



dimensional distribution of constituent elements and quantitative analysis at the subnanometer level.58–62 The three-dimensional APT reconstruction of the needle-shaped sample is shown for the respective Pb (green), Se (orange), and Cu (red) atoms in Figure 3a. The APT results display that Cu atoms appear to be isolated as nanoscale aggregates in the PbSe matrix while Pb and Se atoms are distributed homogeneously throughout the specimen. The magnified view on the Cu-rich region surprisingly uncovers that Cu atoms form discrete single atomic layers and they are randomly distributed in the PbSe lattices with the interval ranging from ~4 – 30 Å (Figure 3b, c and supporting video). A proximity histogram provides the average composition profile across the interface of the Cu-rich region (Figure 3d). Cu concentration shows ~10 at% accumulation at the core and rapidly decreases across it whereas Pb and Se contents display a similar at% throughout the examined area. Observation of discrete single Cu atomic layers implies that interstitial Cu atoms could hop to adjacent interstices and travel a nanoscale distance, which are securely confined in the PbSe matrix at the macroscopic level. This liquid-like behavior strictly restricted in nanoscale can substantially depress lattice thermal conductivity with preserving chemical robustness. However, it is unclear why Cu atoms form nearly parallel layers. A possible reason would be SPS process, which applies high current and temperature across the pellet for densification. According to thermogravimetric analysis, PbSe0.998Br0.002-x%Cu2Se samples do not lose weight up to ~900 K under an Ar flow (Figure S3). They do not show any chemical degradation after the repeated measurements for TE properties. On the contrary, chemical and structural instability of Cu2−xSe materials prohibits its practical thermoelectric applications.55


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Figure 2. Typical atomic resolution Cs-STEM images and direct elemental mapping taken parallel to the zone axis for the PbSe0.998Br0.002-2%Cu2Se sample. (a) Medium-magnification HAADF-STEM image. Strained stripes, enclosed by yellow circles, are embedded in the surrounding PbSe matrix. Inset: FFT image of (a). (b) HAADF-STEM image focusing on the strained stripe. Pb atoms are off-centered from their regular crystallographic site along the zone axis (marked by magenta arrows) inside the stripe. Inset: GPA of (b) showing accumulated strains in and around the stripe. (c) The corresponding ABF-STEM image of (b) clearly shows that interstitial voids in the PbSe lattices are occupied by many small and light atoms (marked by red arrows), which cannot be observed by HAADF-STEM. Inset: the corresponding FFT of (c). (d) Highmagnification atomic resolution HAADF-STEM image taken on the Cu-free region. (e) Atomic resolution elemental mapping by STEM-EDS scanned on (d), revealing ideal PbSe structure. Pb and Se atoms are depicted in green and orange color, respectively. Ideal PbSe structure is drawn for comparison in (f). (g) Elemental mapping scanned on the strained stripe. Cu atoms are shown in red color. Magenta and red arrows indicate the off-centered Pb atoms and interstitial Cu atoms, respectively. The schematic illustration of the local structure is given in (h).



Figure 3. Compositional analysis on nanostructures in the PbSe0.998Br0.002-2%Cu2Se sample by APT. (a) Three-dimensional APT reconstruction of the volume showing the spatial distribution of Pb (green), Se (orange), and Cu (red) atoms. Discrete Cu rich regions are clearly observed in contrast to homogeneous distribution of Pb and Se atoms. (b) Enlarged view on the Cu rich region in (a) (enclosed by the purple circle) revealing the formation of discrete nanoscale single Cu atomic layers. Their interval ranges from ~4 to 30 Å. (c) Magnified view on the single Cu atomic layer marked by the blue rectangle in (b), clearly showing Cu atomic layers. (d) Proximity histogram displaying the concentration profiles of Pb, Se, and Cu atoms across the Cu rich region. The atomic ratio of Pb to Se remains approximately unity while the Cu content sharply decreases from ~10 to 0 at% across the boundary of the Cu-rich region.

3. Charge Transport Properties We performed the temperature-dependent Hall effect measurement for PbSe0.998Br0.002-x%Cu2Se (x = 1 – 7) samples in comparison with the control sample (PbSe0.998Br0.002 (x = 0)). Note that Cu atoms are dissolved into the interstitial holes in the PbSe lattices with exceptionally high solubility up to 7% at room temperature without forming nanoscale precipitates as mentioned earlier. Consequently, PbSe0.998Br0.002x%Cu2Se behaves as single phase to show nearly constant nH with respect to temperature. This phenomenon is in contrast to temperature-dependent nH in PbTe-x%Cu2Te32 and 12 ACS Paragon Plus Environment

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PbCuxSe43 with nanoscale second phase. nH of the latter increases with increasing temperature, and its rate gradually increases with increasing Cu content. The interstitial Cu atoms provide extra charge carriers32,43 to enhance nH significantly from ~2.5 × 1019 for the control to 3.3 – 4.2 ×1019 cm−3 for the samples with x  2 at 723 K (Figure 4a). Lower nH of the x = 1 sample than the control is possibly attributed to Cu atoms either replacing Pb atoms or partially occupying Pb vacancies at this low doping concentration as discussed earlier. The Hall carrier mobility (µH) for PbSe0.998Br0.002-x%Cu2Se (x = 1 – 7) decreases rapidly with increasing temperature, consistent with heavily doped semiconductors with apparent metallic transport behavior (Figure 4b). µH follows a power series of ~T–1.5 (red line) up to ~500 K and ~T–2.5 (dark line) afterward, indicating that the scattering of electron-phonon and phonon-phonon interactions is possibly dominant at low and high temperature, respectively.39 Interestingly, the observed trend is similar to that of n-type Pb0.9975Sb0.0025Se,39 indicating insignificant role of Cu2Se in electron scattering mechanism in PbSe0.998Br0.002-x%Cu2Se. PbSe0.998Br0.002-x%Cu2Se shows lower µH than the control sample at 300 K. Their deviation decreases rapidly with increasing temperature and becomes negligible above 500 K. It is remarkable that heavily Cu2Se doped samples exhibit similar µH with each other in the mid to high temperature range and their µH values are considerably high considering large nH. In general, µH is deteriorated by doping and alloying and decreases with increasing nH due to carrier scattering. Such unusual observation can be attributed to the highly soluble interstitial Cu atoms without giving isolated precipitates and resulting high doping efficiency. Consequentially, a degree of decrease in µH by Cu2Se



introduction is modest considering its doping concentration, indicating weak electron scattering by Cu2Se in this system (Figure S4). Simultaneously realizing high nH, µH, and doping concentration is a core of high TE performance in PbSe0.998Br0.002-x%Cu2Se. Indeed, PbCu0.00375Se43 with limited solubility of Cu shows slightly higher µH and much lower nH, resulting in the poorer electrical conductivity than PbSe0.998Br0.002-x%Cu2Se. Electrical conductivity (σ) for all samples decreases monotonically with increasing temperature, following the trend of µH (Figure 4c). The variation in σ with respect to the mol% of Cu2Se doping is mainly attributed to nH values considering that all samples show similar µH over the entire temperature range. The x = 2 sample, exhibiting a highest peak TE figure merit ZT among the samples, shows slightly higher σ than the control sample above ~400 K despite the high doping level. Its σ is ~2300 and 120 S cm−1 at 300 and 823 K, respectively. The samples with x = 2 – 7 display much higher σ than PbCuxSe (x = 0.00125 – 0.0075),43 verifying weaker electron scattering by interstitial Cu in our samples. For example, σ of the x = 3 sample is ~2950 S cm−1 at 300 K in comparison with ~2000 S cm−1 of PbCu0.00375Se in the literature.43 The temperature-dependent Seebeck coefficient (S) for all samples is negative over the entire range of temperature, confirming their n-type conduction (Figure 4d). Cu2Se doped samples show much higher absolute S (S) than the control and other high performance n-type PbSe materials. For example, the sample with x = 2 shows similar σ and ~15 μV K−1 higher S at 300 K than Pb0.995Sb0.005Se (ZTmax = 1.38).39 Especially, at elevated temperature of 600 – 800 K, the x = 2 sample exhibits similar σ and ~70 μV K−1 larger S than Pb0.9955Sb0.0045Se-6%GeSe (ZTmax = 1.32).56 It is highly remarkable that ntype PbSe0.998Br0.002-x%Cu2Se displays higher S than the optimized state-of-the-art p14 ACS Paragon Plus Environment

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type PbSe-3%CdS at the same temperature range.37 For example, the sample with x = 2 exhibits nearly 100 μV K−1 larger S than PbSe-3%CdS at ~723 K.37 In fact, the record high ZT of 1.6 in the latter is mainly attributed to electronic band structure engineering that induces energetic convergence of its two closely lying valance bands, leading to the significant enhancement in S and power factor.37 In contrast, it is much more challenging to improve Seebeck coefficient and power factor of n-type PbSe by similar strategies because of the lack of second conduction band, leading to its historical underperformance compared to the p-type counterparts. To understand markedly high S, the Pisarenko relations between S and nH for pristine PbSe at 300 K (pink line) and 723 K (blue line) are theoretically plotted (Figure 4e). They are calculated based on a single Kane band model63 assuming acoustic phonon scattering (see Supporting Information for the details). The S values both at 300 and 723 K of n-type PbSe1−yBry64 (y = 0.04% – 0.4%) and PbCu0.00375Se43 from the previous reports fall on the curves, indicating that neither Br nor Cu doping alters conduction bands near the Fermi level. In contrast, PbSe0.998Br0.002-x%Cu2Se shows larger S values than those on the Pisarenko lines at the given nH especially at 723 K, revealing the band parameters such as density-of-states (DOS) effective mass is plausibly manipulated. Considering their operation in the intermediate temperature range, the enhanced S value at high temperature is crucial for TE performance of these materials. Significantly enhanced S coupled with high σ results in greatly improved power factors (PF = S2σ) for PbSe0.998Br0.002-x%Cu2Se (x = 1 − 7). The maximum of PF shifts to higher temperature with the increased Cu2Se doping. It reaches the highest value of ~26 μW cm−1 K−2 at 573 K for the sample with x = 2, which is much larger than those of both



n- and p-type state-of-the-art PbSe thermoelectrics (Figure 4f).39,43 In comparison, the highest PF for n-type PbCu0.0075Se43 and p-type PbSe-3%CdS37 is ~24 and 18 μW cm−1 K−2. Note that high PF is directly related to the high output power of TE power generators, which is a core factor for their broad commercialization.

Figure 4. Charge transport properties of PbSe0.998Br0.002-x%Cu2Se samples (x = 0 – 7). (a) Hall carrier concentration. (b) Hall carrier mobility. The gray and red lines denote the relationship of μH to temperature. (c) Electrical conductivity, (d) Seebeck coefficient. (e) The theoretical Pisarenko plots at 300 K (the red line) and 723 K (the green line) based on a single Kane band model. Experimental Seebeck coefficients of PbSe0.998Br0.002x%Cu2Se samples lie far above the Pisarenko lines at given Hall carrier concentrations at both 300 and 723 K (the red symbols). The experimental data for PbSe1−yBry (y = 0.04% − 0.4%) and PbCu0.00375Se from the previous reports,43,64 falling on the Pisarenko lines, are given for comparison (the black and orange symbols, respectively). (f) Power factor.

4. Electronic Structure Calculations. To understand the origin of enhanced Seebeck coefficient, we performed first principles density functional theory (DFT) calculations for electronic structures for pristine PbSe and PbSe-x%Cu2Se (x = 1, 2, 3). Br is not included in the calculations for the simplicity because it only serves to tune carrier concentration for optimizing n-type conduction. For the supercells involving Cu2Se impurities, one Pb atom is removed and 16 ACS Paragon Plus Environment

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instead two Cu atoms are introduced into the interstitial holes in the PbSe lattices. The distance among three defects are freely released to reflect their random distributions as observed in the STEM and APT studies (Figure S5a). Effective mass of electron (m*) increases from 0.109me for pristine to 0.111me, 0.123me, and 1.023me for 1, 2, and 3% Cu2Se doped PbSe, respectively. It is remarkable that 3% Cu2Se doping increases m* by an order of magnitude, resulting in the almost flat band at the conduction band edge. Cu2Se doping also enlarges the energy band gap from 0.17 eV for pristine to 0.23 and 0.22 eV for the x = 2 and 3 samples, respectively, which excellently matches with our experimental spectroscopy results (Figure 5). These results can be attributed to introducing Cu cations into interstitial sites in the PbSe lattices. They do not directly participate in forming the conduction band edge. However, they perturb the cubic PbSe lattice, thereby decreasing bandwidths. This can lead to the increased m*, flattening the conduction band edge gradually as the degree of Cu doping increases. Note that heavier m* leads to a higher DOS near the Fermi level; and larger band gap reflects higher polarity in charge carriers. These factors are well known to increase Seebeck coefficient.12 Indeed, our DFT calculations show that 2% and 3% Cu2Se doping significantly enhances Seebeck coefficient as expected from their modulated electronic conduction band structures (Figure S6a). These results explain the unusually high Seebeck coefficient in PbSe0.998Br0.002-x%Cu2Se, which greatly deviates from the single band model, shown in the Pisarenko plot, in contrast to the general understanding on ntype PbSe-based materials (Figure 4e). We also modeled the control configuration mimicking the formation of Cu-related precipitates as previously observed in PbCu0.0075Se43 and PbTe-5.5%Cu2Te.32 Two Cu



atoms are located in the vicinity of Pb vacancy. Structural optimization at the DFT level shows that two Cu atoms slightly move toward negatively charged Pb vacancy positions and form Cu-rich clusters locally because of Coulomb interaction (Figure S5b). However, the calculation results from this model give decreased Seebeck coefficients and energy band gaps (Figures S6b and S7), contrary to our experimental observations. This result suggests that high Seebeck coefficients of PbSe0.998Br0.002-x%Cu2Se are plausibly associated with a random distribution of interstitial Cu atoms in the PbSe matrix without forming nanoscale precipitates, consistent with the observations by STEM and APT. Dissolved Cu in the PbSe matrix results in the distinguished effect of Cu2Se doping on charge transport properties of PbSe0.998Br0.002, in comparison with other n-type PbCu0.0075Se43 and PbTe-5.5%Cu2Te32 materials that do not involve modified conduction bands and enhanced Seebeck coefficients. To confirm the validity of our model of choice (1) over the control configuration (2) for the DFT calculations, we compared their formation energy. The calculated formation enthalpy difference, ΔH = H1 – H2, is 1.17, 2.20, and 0.88 eV for the samples with x = 1, 2, and 3, respectively, indicating that the control configuration is energetically more favorable than our model of choice. This result is attributed to energy gain from Coulomb interaction among positively charged interstitial Cu and negatively charged Pb vacancy in the PbSe matrix. However, our model has free energy gain from the entropy term, 2kBT(1 – ln c + ln 2), where c is the total concentration of defects, given configurational entropy. At near room temperature, this entropy term is much smaller than the enthalpy difference. However, the entropy term increases rapidly with rising temperature. Consequently, the population of our model becomes larger at higher


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temperature, which leads to the abnormal enhancement of Seebeck coefficients, consistent with our experimental observations for PbSe0.998Br0.002-x%Cu2Se.

Figure 5. Electronic band structures for PbSe-x%Cu2Se (x = 1 – 3) in comparison with pristine PbSe. EF stands for Fermi energy level.

5. Thermal Transport Properties Figure 6a shows total thermal conductivity (κtot) of PbSe0.998Br0.002-x%Cu2Se (x = 1 − 7) samples as a function of temperature. All Cu2Se doped samples show significantly suppressed κtot compared with the control sample. Their κtot decreases with increasing temperature. It increases with the larger fraction of Cu2Se doping up to x = 3 and decreases afterward (Figure 6a). Since κtot is contributed from charge carriers (κele) and lattice phonons (κlat), the subtraction of κele from κtot in the Wiedeman−Franz relationship (see Supporting Information for the details) reveals the effect of Cu2Se doping on thermal transport. Figure 6b clearly shows that Cu2Se doping substantially suppresses κlat for all samples over the entire temperature range. At 300 K, κlat decreases from ~1.8 W m−1 K−1 for the control sample to ~1.2 W m−1 K−1 for the x = 2 sample. At high temperature, phonon scattering by Cu2Se doping is more effective to give a remarkable decrease for the same samples from ~1.4 W m−1 K−1 to ~0.4 W m−1 K−1 at 723 K. The latter value approaches the theoretical lower bound of κlat ~0.37 W m−1 K−1, calculated by a



Debye−Callaway model.42 To understand the origin of extremely low thermal conductivity, we calculated phonon band structure for pristine PbSe and PbSe-3%Cu2Se based on force constants from first principles calculations. Since their κlat decreases with increasing temperature, we only focus on the origin of suppressed κlat at near room temperature. Our DFT calculation identified the control configuration as a room temperature phase in terms of the free energy. As a consequence, we mainly compare phonon dispersions of pristine PbSe and PbSe-3%Cu2Se in the control configuration. Figures 7a and b show phonon band structure (left panel) and total DOS (right panel) for pristine PbSe and PbSe3%Cu2Se, respectively. The calculated phonon band structures clearly demonstrate that three acoustic modes are remarkably softened in PbSe-3%Cu2Se. For pristine PbSe, the longitudinal acoustic (LA) mode spans up to 70 cm−1 and the transverse acoustic (TA) modes up to 45 cm−1. In contrast, acoustic phonon modes of PbSe-3%Cu2Se are severely softened near M and R points. Their maximum frequencies decrease to 30 and 20 cm−1 for the LA and TA modes, respectively. As a result, a shift in phonon DOS occurs near 30 cm−1 in PbSe-3%Cu2Se. This softening can be attributed to distortion of coordination environments of Pb atoms. Figure 7c shows the projected DOS (PDOS) of PbSe3%Cu2Se, where Pb atoms are categorized into two different classes: PbNN is next nearest neighbors of Cu atoms and PbFAR is the others distant from Cu atoms. The phonon PDOS of PbSe-3%Cu2Se clarifies that PbNN atoms, presented in Figure 7d, mainly contribute to the observed softening. In this atomic configuration, doped-Cu atoms are displaced from the center of the interstitial voids shown in Figure 1a, approaching to the vicinity of negatively charged Pb vacancies due to Coulomb interaction. Consequently, the Cu-Se


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bond distance decreases. Such off-centered Cu atoms induce a local strain field, subsequently distorting atomic environment of PbNN to give long and short bond distances between Pb and Se atoms. In general, phonon-phonon scattering is enhanced by such substantial softening in phonon modes occurring at low-frequency regime, contributing to ultralow thermal conductivity of PbSe0.998Br0.002-x% Cu2Se.

Figure 6. (a) Total (κtot) and (b) lattice thermal conductivity (κlat) of PbSe0.998Br0.002x%Cu2Se (x = 0 – 7) with respect to temperature.



Figure 7. Phonon band structure (the left panel) and total density of states (DOS) (the right panel) for (a) pristine PbSe and (b) PbSe-3%Cu2Se. The transverse acoustic (TA) and the longitudinal acoustic (LA) phonon branches are presented by blue/cyan and red solid lines, respectively. (c) The projected phonon DOS of PbSe-3%Cu2Se. Blue, purple, black, and green curves stand for projection to Cu, PbNN, PbFAR, and Se atoms, respectively. PbNN denotes next nearest neighbors of Cu atoms and PbFAR corresponds to all the others that are distant from Cu atoms. (d) Illustration of the defect structure. Blue, purple, black, and green spheres correspond to Cu, PbNN, PbFAR, and Se atoms.

6. Figure of merit. The temperature-dependent TE figure of merit ZT for PbSe0.998Br0.002-x%Cu2Se samples is shown in Figure 8a. Cu2Se doping markedly enhances ZT for all samples from ~0.6 of pristine PbSe0.998Br0.002 to larger than 1.2. PbSe0.998Br0.002-2%Cu2Se exhibits the highest peak ZT (ZTmax) of ~1.8 at 723 K and average ZT (ZTave) of ~1.1 between 300 K and 823 K. The achieved ZTmax and ZTave are record high values reported to date for nand p-type PbSe-based TE materials as well as n-type tellurium-free polycrystalline TE 22 ACS Paragon Plus Environment

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materials (Figure 8b). The ZTmax of ~1.8 matches the highest ZT value of n-type PbTebased materials such as AgPbmSbTe2+m (LAST-m)29 and PbTe0.998I0.002-3%Sb.30 Remarkably, this outstanding TE performance is ascribed to the enhanced Seebeck coefficient by electronic band structure manipulation and consequentially improved power factor, synergistically coupled with ultralow thermal conductivity. All these favorable effects are realized by dissolving Cu2Se into the PbSe lattices. Reproducibility and reversibility of charge carrier and thermal transport properties are of pivotal importance in TE devices and their practical applications. We prepared three different specimens of PbSe0.998Br0.002-2%Cu2Se from the independent synthesis batches. The core parameters of σ, S, and tot and ZT are highly reproducible (Figure S8). Their hysteresis upon the repeated heating and cooling cycles are also negligible within the range of instrumental errors, verifying high reversibility of TE performance on the repeated heating cycles (Figure S9). These results together with the TGA data (Figure S3) confirm high thermal and chemical robustness of this material.



Figure 8. (a) ZT values of PbSe0.998Br0.002-x%Cu2Se (x = 0 – 7) with respect to temperature. (b) A ZT comparison of the x = 2 sample with the state-of-the-art n-type PbSe thermoelectric materials of Pb0.95Sb0.033Se,42 Pb0.995Sb0.005Se,39 PbCu0.00375Se,43 and Pb0.95Sb0.033Se0.6Te0.4.44

■ Conclusion. Cu2Se doping extraordinarily enhances Seebeck coefficient of n-type PbSe above the theoretical expectations based on a single Kane band model. First principles calculations suggest that Cu2Se increases the effective mass of electron and flattens the conduction band minimum as well as widens the energy band gap of PbSe, giving markedly improved Seebeck coefficient. This finding is in striking contrast to the general understanding that n-type PbSe-based materials are unavailable for electronic band


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structure engineering to enhance Seebeck coefficient due to the absence of high-lying second conduction band. Cu2Se also uniquely dissolves into interstitial voids in the PbSe lattices to form individual atomic layers in the matrix. Consequently, it provides extra charge carriers without damaging carrier mobility above ~500 K, thereby giving slightly enhanced electrical conductivity. Resulting powder factor of PbSe0.998Br0.002-2%Cu2Se is larger than ~21 μW cm−1 K−2 from 423 to 723 K with the highest ~26 μW cm−1 K−2 at 573 K. These values exceed those of the state-of-the art p-type PbSe-based materials that benefit from valence band convergence and consequently enhanced Seebeck coefficient and power factor. Cu2Se doping induces ultralow lattice thermal conductivity of ~0.4 W m−1 K−1 at 773 K in PbSe0.998Br0.002-2%Cu2Se. First principles phonon dispersion calculations reveal significantly softened acoustic phonon modes at low frequency by the Cu2Se doping. Synergistically improving charge carrier transport and depressing thermal conductivity in PbSe0.998Br0.002-2%Cu2Se enable record high peak ZT of ~1.8 at 723 K and average ZT of ~1.1 from 300 to 823 K among all the n- and p-type PbSe-based materials. This material outperforms any class of tellurium-free n-type polycrystalline thermoelectric materials and is on par with the best performing n-type PbTe-based materials. The achievement of this work is a new milestone for tellurium-free materials, which outperforms traditional PbTe-based materials in the intermediate temperature range, opening a new era of economically viable and highly efficient thermoelectric power generation.



■ Experimental Section Synthesis and Sample Preparation. Reagents including Pb, Cu, Se (99.999%, American Elements), and PbBr2 (99.999%, Sigma-Aldrich) were used as received. Ingots (~12 g) with the nominal compositions PbSe0.998Br0.002-x%Cu2Se (x = 0, 1, 2, 3, 5, 7) were synthesized by reacting an appropriate molar ratio of starting reagents in an evacuated fused silica tube (~105 Torr) at 1423 K for 6h, followed by quenching to ice water. The obtained ingots were substantially annealed at 1023 K for 48h and naturally cooled to room temperature. The products were hand-ground by an agate mortar and pestle to fine powder in an Ar-filled glove box. The resulting powder was densified at 823 K for 5 min under an axial pressure of 50 MPa in a vacuum using spark plasma sintering (SPS) (SPS211Lx, Fuji Electronic Industrial Co., Japan). After SPS, a cylindrical sample with a thickness of ~12.7 mm and a diameter of 12 mm was obtained. The density of the samples was calculated from the geometrical dimensions and masses. For Cu2Se doped samples, it ranges from 98.8 to 99.7% of the theoretical density for Cu2Se doped specimens in comparison with 97.5% for PbSe0.998Br0.002 (Table S1). X-ray Powder Diffraction (XRD). XRD patterns for all samples were collected using Cu Kα ( = 1.5418 Å) graphite monochromatized radiation on a SmartLab Rigaku powder X-ray diffractometer operating at 40 kV and 20 mA. Infrared Spectroscopy. To measure the electronic energy gap, ingots with the composition of PbSe-x%Cu2Se (x = 0, 1, 2, 3) were prepared by the same procedure as described above. Br was excluded to avoid spectroscopic interference by free carriers. Their infrared diffuse reflectance spectra were collected on a Bruker Vertex 70 FTIR spectrometer in the mid-IR range (6000 − 400 cm−1) at ambient temperature. Reflectance 26 ACS Paragon Plus Environment

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data were converted to absorption data using the Kubelka-Munk equation.65,66 Charge Carrier Transport. To characterize charge transport properties, SPS processed samples were cut and polished into various shapes and dimensions. The Seebeck coefficient and electrical conductivity were simultaneously measured for bar-shaped samples with the dimension of ~12 × 3 × 3 mm3 by an ULVAC-RIKO ZEM-3 instrument under a low-pressure He atmosphere from room temperature to 823 K. Hall effect measurements as a function of temperature were performed on a Lakeshore 8407 system from 300 to 773 K under an ultrahigh vacuum (2 in PhaseSeparated PbTe0.7S0.3. Nat. Commun. 2014, 5, 4515. Xiao, Y.; Wu, H.; Li, W.; Yin, M.; Pei, Y.; Zhang, Y.; Fu, L.; Chen, Y.; Pennycook, S. J.; Huang, L.; He, J.; Zhao, L.-D. Remarkable Roles of Cu to Synergistically Optimize Phonon and Carrier Transport in n-Type PbTe-Cu2Te. J. Am. Chem. Soc. 2017, 139, 18732. Xiao, Y.; Wu, H.; Cui, J.; Wang, D.; Fu, L.; Zhang, Y.; Chen, Y.; He, J.; Pennycook, S. J.; Zhao, L.-D. Realizing High Performance n-Type PbTe by Synergistically Optimizing Effective Mass and Carrier Mobility and Suppressing Bipolar Thermal Conductivity. Energy Environ. Sci. 2018. Rattner, A. S.; Garimella, S. Energy Harvesting, Reuse and Upgrade to Reduce Primary Energy Usage in the USA. Energy 2011, 36, 6172. Amatya, R.; Ram, R. J. Trend for Thermoelectric Materials and Their Earth Abundance. J. Electron. Mater. 2012, 41, 1011. Chasapis, T. C.; Lee, Y.; Hatzikraniotis, E.; Paraskevopoulos, K. M.; Chi, H.; Uher, C.; Kanatzidis, M. G. Understanding the Role and Interplay of Heavy-Hole and Light-Hole Valence Bands in the Thermoelectric Properties of PbSe. Phys. Rev. B: Condens. Matter Mater. Phys. 2015, 91, 085207. Zhao, L.-D.; Hao, S.; Lo, S.-H.; Wu, C.-I.; Zhou, X.; Lee, Y.; Li, H.; Biswas, K.; Hogan, T. P.; Uher, C.; Wolverton, C.; Dravid, V. P.; Kanatzidis, M. G. High Thermoelectric Performance via Hierarchical Compositionally Alloyed Nanostructures. J. Am. Chem. Soc. 2013, 135, 7364. Lee, Y.; Lo, S.-H.; Androulakis, J.; Wu, C.-I.; Zhao, L.-D.; Chung, D.-Y.; Hogan, T. P.; Dravid, V. P.; Kanatzidis, M. G. High-Performance Tellurium-Free Thermoelectrics: All-Scale Hierarchical Structuring of p-Type PbSe-MSe Systems (M = Ca, Sr, Ba). J. Am. Chem. Soc. 2013, 135, 5152. Lee, Y.; Lo, S.-H.; Chen, C.; Sun, H.; Chung, D.-Y.; Chasapis, T. C.; Uher, C.; Dravid, V. P.; Kanatzidis, M. G. Contrasting Role of Antimony and Bismuth Dopants on the Thermoelectric Performance of Lead Selenide. Nat. Commun. 2014, 5, 3640. Androulakis, J.; Todorov, I.; He, J.; Chung, D.-Y.; Dravid, V. P.; Kanatzidis, M. G. Thermoelectrics from Abundant Chemical Elements: High-Performance Nanostructured PbSe–PbS. J. Am. Chem. Soc. 2011, 133, 10920. Androulakis, J.; Lee, Y.; Todorov, I.; Chung, D.-Y.; Kanatzidis, M. G. HighTemperature Thermoelectric Properties of n-Type PbSe Doped with Ga, In,and Pb. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 82, 035204. Chen, Z.; Ge, B.; Li, W.; Lin, S.; Shen, J.; Chang, Y.; Hanus, R.; Snyder, G. J.; Pei, Y. Vacancy-Induced Dislocations within Grains for High-Performance PbSe Thermoelectrics. Nat. Commun. 2017, 8, 13828. You, L.; Liu, Y.; Li, X.; Nan, P.; Ge, B.; Jiang, Y.; Luo, P.; Pan, S.; Pei, Y.; Zhang, W.; Snyder, G. J.; Yang, J.; Zhang, J.; Luo, J. Boosting the Thermoelectric Performance of PbSe through Dynamic Doping and Hierarchical Phonon 32 ACS Paragon Plus Environment

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Scattering. Energy Environ. Sci. 2018, 11, 1848. Zhou, C.; Lee, Y. K.; Cha, J.; Yoo, B.; Cho, S.-P.; Hyeon, T.; Chung, I. Defect Engineering for High Performance n-Type PbSe Thermoelectrics. J. Am. Chem. Soc. 2018, 140, 9282. Wu, H.; Chang, C.; Feng, D.; Xiao, Y.; Zhang, X.; Pei, Y.; Zheng, L.; Wu, D.; Gong, S.; Chen, Y.; He, J.; Kanatzidis, M. G.; Zhao, L.-D. Synergistically Optimized Electrical and Thermal Transport Properties of SnTe via Alloying High-Solubility MnTe. Energy Environ. Sci. 2015, 8, 3298. Slade, T. J.; Grovogui, J. A.; Hao, S.; Bailey, T. P.; Ma, R.; Hua, X.; Gueuen, A.; Uher, C.; Wolverton, C.; Dravid, V. P.; Kanatzidis, M. G. Absence of Nanostructuring in NaPbmSbTem+2 : Solid Solutions with High Thermoelectric Performance in the Intermediate Temperature Regime. J. Am. Chem. Soc. 2018, 140, 7021. Shannon, R. D. Revised Effective Ionic Radii and Systematic Studies of Interatomic Distances in Halides and Chalcogenides. Acta Crystallogr. Sect. A 1976, 32, 751. Pei, Y.; Chang, C.; Wang, Z.; Yin, M.; Wu, M.; Tan, G.; Wu, H.; Chen, Y.; Zheng, L.; Gong, S.; Zhu, T.; Zhao, X.; Huang, L.; He, J.; Kanatzidis, M. G.; Zhao, L.-D. Multiple Converged Conduction Bands in K2Bi8Se13: A Promising Thermoelectric Material with Extremely Low Thermal Conductivity. J. Am. Chem. Soc. 2016, 138, 16364. Shi, X.; Yang, J.; Salvador, J. R.; Chi, M.; Cho, J. Y.; Wang, H.; Bai, S.; Yang, J.; Zhang, W.; Chen, L. Multiple-Filled Skutterudites: High Thermoelectric Figure of Merit through Separately Optimizing Electrical and Thermal Transports. J. Am. Chem. Soc. 2011, 133, 7837. Zheng, Z.; Su, X.; Deng, R.; Stoumpos, C.; Xie, H.; Liu, W.; Yan, Y.; Hao, S.; Uher, C.; Wolverton, C.; Kanatzidis, M. G.; Tang, X. Rhombohedral to Cubic Conversion of GeTe via MnTe Alloying Leads to Ultralow Thermal Conductivity, Electronic Band Convergence and High Thermoelectric Performance. J. Am. Chem. Soc. 2018, 140, 2673. Chung, D.-Y.; Choi, K.-S.; Iordanidis, L.; Schindler, J. L.; Brazis, P. W.; Kannewurf, C. R.; Chen, B.; Hu, S.; Uher, C.; Kanatzidis, M. G. High Thermopower and Low Thermal Conductivity in Semiconducting Ternary K-Bi-Se Compounds . and Their Sb Analogues. Chem. Mater. 1997, 9, 3060. Rogl, G.; Grytsiv, A.; Rogl, P.; Peranio, N.; Bauer, E.; Zehetbauer, M.; Eibl, O. NType Skutterudites (R,Ba,Yb)yCo4Sb12 (R = Sr, La, Mm, DD, SrMm, SrDD) Approaching ZT ≈ 2.0. Acta Mater. 2014, 63, 30. Christensen, M.; Abrahamsen, A. B.; Christensen, N. B.; Juranyi, F.; Andersen, N. H.; Lefmann, K.; Andreasson, J.; Bahl, C. R. H.; Iversen, B. B. Avoided Crossing of Rattler Modes in Thermoelectric Materials. Nat. Mater. 2008, 7, 811. Wang, J.; He, Y.; Mordvinova, N. E.; Lebedev, O. I.; Kovnir, K. The Smaller the Better : Hosting Trivalent Rare- Earth Guests in Cu–P Clathrate Cages. Chem 2018, 4, 1. Olvera, A. A.; Moroz, N. A.; Sahoo, P.; Ren, P.; Bailey, T. P.; Page, A. A.; Uher, C.; Poudeu, P. F. P. Partial Indium Solubility Induces Chemical Stability and Colossal Thermoelectric Figure of Merit in Cu2Se. Energy Environ. Sci. 2017, 10, 33 ACS Paragon Plus Environment


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1668. Luo, Z.; Hao, S.; Zhang, X.; Hua, X.; Cai, S.; Tan, G.; Bailey, T.; Ma, R.; Uher, C.; Wolverton, C.; Dravid, V. P.; Yan, Q.; Kanatzidis, M. G. Soft Phonon Modes from Off-Center Ge Atoms Lead to Ultralow Thermal Conductivity and Superior Thermoelectric Performance in n-Type PbSe-GeSe. Energy Environ. Sci. 2018, DOI: 10.1039/c8ee01755g Ravich, Iu. I. Semiconducting Lead Chalcogenides; 1970. Yu, Y.; Zhang, S.; Mio, A. M.; Gault, B.; Sheskin, A.; Scheu, C.; Raabe, D.; Zu, F.; Wuttig, M.; Amouyal, Y.; Cojocaru-Mirédin, O. Ag-Segregation to Dislocations in PbTe-Based Thermoelectric Materials. ACS Appl. Mater. Interfaces 2018, 10, 3609. Gault, B. A Brief Overview of Atom Probe Tomography Research. Appl. Microsc. 2016, 46, 117. Kuzmina, M.; Herbig, M.; Ponge, D.; Sandlöbes, S.; Raabe, D. Linear Complexions: Confined Chemical and Structural States at Dislocations. Science. 2015, 349, 1080. Miller, M. K.; Russell, K. F. Atom Probe Specimen Preparation with a Dual Beam SEM/FIB Miller. Ultramicroscopy 2007, 107, 761. Cagnoni, M.; Führen, D.; Wuttig, M. Thermoelectric Performance of IV – VI Compounds with Octahedral-Like Coordination : A Chemical-Bonding Perspective. Adv. Mater. 2018, 30, 1801787. Wu, D.; Zhao, L.; Hao, S.; Jiang, Q.; Zheng, F.; Doak, J. W.; Wu, H.; Chi, H.; Gelbstein, Y.; Uher, C.; Wolverton, C.; Kanatzidis, M. G.; He, J. Origin of the High Performance in GeTe-Based Thermoelectric Materials upon Bi2Te3 Doping. J. Am. Chem. Soc. 2014, 136, 11412. Wang, H.; Pei, Y.; LaLonde, A. D.; Snyder, G. J. Weak Electron-Phonon Coupling Contributing to High Thermoelectric Performance in n-Type PbSe. Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 9705. Wendlandt, W. W.; Hecht, H. G. Reflectance Spectroscopy; John Wiley & Sons Inc, 1966. Kortüm, G. Reflectance Spectroscopy: Principles, Methods, Applications; Springer-Verlag, 1969. Blachnik, R.; Igel, R. Thermodynamische Eigenschaften von IV-VI-Verbindungen: Bleichalkogenide. Z. Naturforsch. B: J. Chem. Sci 1974, 29, 625.


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ToC use only



Figure 1. (a) Illustration of the PbSe structure with Cu atoms residing at interstitial voids. It is at the center of the cube comprising each four alternating Pb and Se atoms. From another view, it is shared by Pb tetrahedron (blue) and Se tetrahedron (orange), which interpenetrate with each other. (b) Powder XRD patterns and (c) lattice parameter with respect to the Cu2Se content for the PbSe0.998Br0.002-x%Cu2Se (x = 0 – 7) samples. (d) Electronic absorption spectra of the PbSe-x%Cu2Se samples (x = 0, 1, 2 and 3). 404x310mm (96 x 96 DPI)

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Figure 2. Typical atomic resolution Cs-STEM images and direct elemental mapping taken parallel to the zone axis for the PbSe0.998Br0.002-2%Cu2Se sample. (a) Medium-magnification HAADF-STEM image. Strained stripes, enclosed by yellow circles, are embedded in the surrounding PbSe matrix. Inset: FFT image of (a). (b) HAADF-STEM image focusing on the strained stripe. Pb atoms are off-centered from their regular crystallographic site along the zone axis (marked by magenta arrows) inside the stripe. Inset: GPA of (b) showing accumulated strains in and around the stripe. (c) The corresponding ABF-STEM image of (b) clearly shows that interstitial voids in the PbSe lattices are occupied by many small and light atoms (marked by red arrows), which cannot be observed by HAADF-STEM. Inset: the corresponding FFT of (c). (d) Highmagnification atomic resolution HAADF-STEM image taken on the Cu-free region. (e) Atomic resolution elemental mapping by STEM-EDS scanned on (d), revealing ideal PbSe structure. Pb and Se atoms are depicted in green and orange color, respectively. Ideal PbSe structure is drawn for comparison in (f). (g) Elemental mapping scanned on the strained stripe. Cu atoms are shown in red color. Magenta and red arrows indicate the off-centered Pb atoms and interstitial Cu atoms, respectively. The schematic illustration of the local structure is given in (h). 391x178mm (96 x 96 DPI)



Figure 3. Compositional analysis on nanostructures in the PbSe0.998Br0.002-2%Cu2Se sample by APT. (a) Three-dimensional APT reconstruction of the volume showing the spatial distribution of Pb (green), Se (orange), and Cu (red) atoms. Discrete Cu rich regions are clearly observed in contrast to homogeneous distribution of Pb and Se atoms. (b) Enlarged view on the Cu rich region in (a) (enclosed by the purple circle) revealing the formation of discrete nanoscale single Cu atomic layers. Their interval ranges from ~4 to 30 Å. (c) Magnified view on the single Cu atomic layer marked by the blue rectangle in (b), clearly showing Cu atomic layers. (d) Proximity histogram displaying the concentration profiles of Pb, Se, and Cu atoms across the Cu rich region. The atomic ratio of Pb to Se remains approximately unity while the Cu content sharply decreases from ~10 to ~0 at% across the boundary of the Cu-rich region. 391x227mm (96 x 96 DPI)


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Figure 4. Charge transport properties of PbSe0.998Br0.002-x%Cu2Se samples (x = 0 – 7). (a) Hall carrier concentration. (b) Hall carrier mobility. The gray and red lines denote the relationship of μH to temperature. (c) Electrical conductivity, (d) Seebeck coefficient. (e) The theoretical Pisarenko plots at 300 K (the red line) and 723 K (the green line) based on a single Kane band model. Experimental Seebeck coefficients of PbSe0.998Br0.002-x%Cu2Se samples lie far above the Pisarenko lines at given Hall carrier concentrations at both 300 and 723 K (the red symbols). The experimental data for PbSe1−yBry (y = 0.04% − 0.4%) and

PbCu0.00375Se from the previous reports,43,64 falling on the Pisarenko lines, are given for comparison (the black and orange symbols, respectively). (f) Power factor. 256x129mm (300 x 300 DPI)



Figure 5. Electronic band structures for PbSe-x%Cu2Se (x = 1 – 3) in comparison with pristine PbSe. EF stands for Fermi energy level. 272x134mm (300 x 300 DPI)


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Figure 6. (a) Total (κtot) and (b) lattice thermal conductivity (κlat) of PbSe0.998Br0.002-x%Cu2Se (x = 0 – 7) with respect to temperature. 191x296mm (96 x 96 DPI)



Figure 7. Phonon band structure (the left panel) and total density of states (DOS) (the right panel) for (a) pristine PbSe and (b) PbSe-3%Cu2Se. The transverse acoustic (TA) and the longitudinal acoustic (LA) phonon branches are presented by blue/cyan and red solid lines, respectively. (c) The projected phonon DOS of PbSe-3%Cu2Se. Blue, purple, black, and green curves stand for projection to Cu, PbNN, PbFAR, and Se atoms, respectively. PbNN denotes next nearest neighbors of Cu atoms and PbFAR corresponds to all the others that are distant from Cu atoms. (d) Illustration of the defect structure. Blue, purple, black, and green spheres correspond to Cu, PbNN, PbFAR, and Se atoms. 202x275mm (300 x 300 DPI)


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Figure 8. (a) ZT values of PbSe0.998Br0.002-x%Cu2Se (x = 0 – 7) with respect to temperature. (b) A ZT comparison of the x = 2 sample with the state-of-the-art n-type PbSe thermoelectric materials of Pb0.95Sb0.033Se,42 Pb0.995Sb0.005Se,39 PbCu0.00375Se,43 and Pb0.95Sb0.033Se0.6Te0.4.44 192x301mm (96 x 96 DPI)


High Performance n-type PbSe-Cu2Se Thermoelectrics through

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